I think that .biz has potential, but I like .us better.
I would invest in some .biz myself, but I'm afraid that I'll get caught up in the registry rules. According to the registry rules, you're not supposed to buy .biz domains just to sell them off.
In order to meet the registry rules, there's gotta be some development of the .biz domain hooked on with some type of a business.
If the registry finds a violator, the registry could bring about a forced surrender of the domain name. Although they may not be enforcing this rule right now, it could be enforced at any time.
If they didn't have that rule, then I would invest in some .biz domains.